There are thermometers that show a driver the outside air temperature. They comprise an exterior sensor wired to a dial, which is incorporated into a dash board. However, because such equipment is relatively expensive and consumes space on the dash board, manufacturers install them only on more expensive vehicles, and it is difficult for the user to install one of these types of thermometers after purchase.
One solution to the above-mentioned problem is a thermometer in which the case includes, with respect to the dial, a transparent face surrounded by an adhesive surface, allowing the case to be fastened to the outside surface of the vehicle's windshield. This remedy has serious disadvantages, however. First, the thermometer is influenced by the interior temperature of the vehicle, due to radiation and/or conduction, so that it often gives a false reading. Second, because it would be dangerous to place the thermometer facing the driver, where it would be in the way of his or her view of the road, the thermometer must be placed to the side with respect to the field of vision, and it is thus difficult to read the dial.